Pull content into topics (topicpull)

The topicpull step pulls content into <xref> and <link> elements. This step is
implemented in XSLT.

If an <xref> element does not contain link text, the target is examined and the link text is pulled.
For example, a reference to a topic pulls the title of the topic; a reference to a list item pulls the number of
the item. If the <xref> element references a topic that has a short description, and the <xref> element
does not already contain a child <desc> element, a <desc> element is created that contains the text from
the topic short description.

The process is similar for <link> elements. If the <link> element does
not have a child <linktext> element, one is created with the appropriate link text. Similarly, if the
<link> element does not have a child <desc> element, and the short description of the target can be
determined, a <desc> element is created that contains the text from the topic short
description.